—An airplane carries 500 bricks. One of the bricks falls out. How many bricks are left in the airplane?
—This is easy: 499!
—Correct. Next question. How do you put a giraffe into a refrigerator?
—Open the refrigerator, put in the giraffe, and close the refrigerator door.
—Good, next. How do you put an elephant into a refrigerator?
—Open the refrigerator, take out the giraffe, put in the elephant and close the door.
—Correct. The Lion King is hosting his birthday party. All the animals come to congratulate him—except one. Why?
—The elephant couldn’t come because it is in the refrigerator.
—Fantastic, next. A man needs to cross a river inhabited by crocodiles and he doesn’t have a boat. What should he do?
—He can just swim: all the crocodiles are attending Lion King’s birthday party.
—Amazing! The last question: The man swims across the river, and dies. What happened?

sheldon:

M:

me:

It’s funny for a specific type of person… I’ll leave the proof that such a person is likely to obtain/have obtained a STEM degree to the reader.
Or, put differently, the clue to the joke is the realization that the unrelated clauses in it share state.